Photolithographic processing includes forming a patterned image of radiation (i.e., actinic energy) onto a photosensitive material such as photoresist. The radiation alters exposed regions relative to unexposed regions. Subsequent development of the photosensitive material selectively removes the exposed regions relative to the unexposed regions, or vice-versa, to thereby pattern the photosensitive material on the substrate. Material underlying the patterned photosensitive material may then be processed through openings formed in the patterned photosensitive material.
The patterned image of radiation is formed by passing radiation through a mask and onto the photosensitive material. The mask itself may be formed by photolithography or other manners, for example using an E-beam mask writer. Some masks and corresponding applied energy are sufficient in size and quantity to pattern an entire wafer in a single exposure. Other masks (e.g., a reticle) only pattern a single small area of the wafer in a single exposure. The reticle and/or substrate are then stepped relative to one another to pattern other areas of the substrate until all desired areas of the substrate have been patterned with the mask.
The patterned image formed in the photosensitive material will have individual features of various size, shape, and spacing relative one another. Undesired variation from targeted size, shape, and/or spacing can occur within an exposure field created on the substrate using a mask. This is commonly referred to as intra-field variation, and can be caused by feature variations in the mask and imperfections in the projection tool in which the mask is received. The substrate feature variations may be in the form of variable critical dimension (CD) values in one or more directions relative to commonly sized, shaped, and spaced features. Additionally or alternately, the variations may be in the form of CD bias relative to differently sized, shaped, and/or spaced features. Existing techniques for improving intra-field substrate feature variation include one or both of exposure tool based techniques and mask based techniques. Exposure tool techniques typically use on-scan dose tuning combined with intra-slit intensity profiling. Dose tuning during stage movement can provide a change in CD values in the direction of scan. Change of CD values orthogonally to the scan direction can be achieved by optical filters within the illumination system or by a local long-slit variation of the slit height. Mask based techniques may include providing sub-resolution mask features adjacent certain of the resolving mask features. (As used herein, “mask feature” refers to a feature formed on the mask in question, whereas “substrate feature” refers to a feature that is formed on a substrate using the mask.) Mask based techniques may also include attenuating the light intensity by creating micro-holes in the mask substrate beneath the mask features
Needs remain in fabrication of masks where undesired intra-field CD or other feature variation is minimized.